The invention relates to a high-frequency oscillator whose frequency is modulated linearly around a central frequency by means of a variable capacitance diode as a function of a control voltage applied to the terminals of this diode, the oscillator comprising a power transistor whose matching impedances on its input and output electrodes are provided by lines calculated such as to ensure a maximum gain at the central frequency.
The oscillator according to the invention provides an output power from 0.1 to 1 Watt at an operating frequency of some 5 GHz, and is used in all F.M. systems, and more specifically in apparatuses such as radio-altimeter sources or proximity fuses, automatic range finders and long-range radar. In these several applications a proper linearity of the frequency-control voltage characteristic and a very good insulation from noise are required. To satisfy this insulation condition the oscillator must have a high external Q factor.
High-frequency oscillators of the type defined in the opening paragraph are disclosed in, for example, U.S. Pat. No. 4,310,809 which describes an arrangement with negative resistance realized on the basis of a bipolar transistor, and whose frequency variation is controlled by varactors arranged in parallel in the modulation circuit. Oscillators of this type which are of current usage in the field of high-frequencies provide a weak output power, which in the present case, have a value of 10 mW at 8.5 GHz. A method of reducing the sensitivity to exterior influences of such an oscillator consists of providing an insulator at its output.
A further method applicable to a high-power oscillator of the fed-back amplifier type, for example, consists in arranging an attenuator or a directive coupler at the output of this oscillator, but the external Q will only be increased as regards its power ratio.